The invention is based on a circuit for generating a dynamic focusing voltage for a picture tube in accordance with the preamble of claim 1. A circuit of this type is disclosed by WO 93/06688.
In a television receiver, a high focusing DC voltage of the magnitude of 7 kV is applied to the focusing electrode of the picture tube, which voltage effects extensive focusing of the luminous spot generated by the electron beam on the screen over the entire screen area.
In new types of cathode ray tubes having larger deflection angles or screens, the change in the valuexe2x80x94required for optimum focusingxe2x80x94of the focusing voltage in the horizontal direction and in the vertical direction over the screen area is so large that adequate focusing over the entire screen area is not possible with a fixed focusing DC voltage. The focusing voltage must then change dynamically in accordance with the respective position of the luminous spot on the screen in the horizontal direction and in the vertical direction.
For this purpose, it is known to add a horizontal-frequency and/or vertical-frequency dynamic correction voltage via AC voltage coupling to the static focusing DC voltage. The focusing voltage effective on the focusing electrode then comprises a high DC voltage component and an AC voltage component in the form of the abovementioned dynamic focusing voltage. Good matching of the respective focusing voltage to the optimum values for focusing can be achieved with a parabolic correction voltage in the horizontal direction and in the vertical direction.
For the circuit for generating a line-frequency, parabolic correction voltage, there are, in principle, three different circuit types:
In the case of a first circuit type, the primary winding of the transformer operating as a current transformer is connected in series with the deflection current, in particular in series with the deflection coil and the so-called S capacitor or tangent capacitor. A sawtooth-waveform current flows in the secondary winding on account of the sawtooth-waveform deflection current in the primary winding, and is integrated into a parabolic voltage by the capacitor.
In the case of a second circuit type, the transformer serves as a voltage transformer, the voltage across the S capacitor or so-called tangent capacitor being fed to the primary winding.
In the case of a third circuit type, described in WO 93/06688, the primary winding of the transformer is fed with the line flyback pulse, and the transformer is deliberately designed with loose coupling between primary winding and secondary winding. This loose coupling has the following purpose: in order to convert the flyback pulse into a parabolic voltage, the flyback pulse has to be integrated twice. The first integration converts the flyback pulse into a sawtooth-waveform current, while the second integration integrates the sawtooth-waveform current into the parabolic voltage. In this case, the inductance required for the first integration in the series path is formed by the deliberately increased leakage inductance of the transformer.
In the very latest picture tubes, which operate for example with a so-called Eureka gun, also called MR gun, adequate focusing can no longer be achieved with a parabolic correction voltage. It has been shown that, on account of the special gun optical system and the large radius of curvature of the screen of these tubes, a correction voltage which is less pointed and wider at the vertex than a parabola is advantageous. This new form of correction voltage approximately corresponds to the cross section through the centre of a commercially available bathtub and is therefore designated hereinafter by bathtub-waveform correction voltage. A correction voltage of this type can no longer be generated by simple integration of a sawtooth-waveform current.
It is desirable, therefore, to provide a simple circuit with which the abovementioned bathtub-waveform horizontal-frequency correction voltage can be generated from a line-frequency current or a line-frequency voltage. The invention specified in claim 1 is suitable for this purpose. Advantageous embodiments and developments of the invention are specified in the subclaims.
The invention can be applied to the first and third circuit types described. In the case of the invention, in order to generate the bathtub-waveform correction voltage, a frequency-dependent network is provided in parallel with the capacitor across the secondary winding of the transformer supplying the correction voltage, which network is tuned in such a way that it effects an additional sinusoidal current with a period duration approximately equal to the line trace period.
The circuit according to the invention is based on the following effect. The secondary winding of the transformer acts as a current source which effects a sawtooth-waveform current in the combination comprising the capacitor and the abovementioned network. The resulting current in the capacitor is the difference between the sawtooth-waveform current from the secondary winding and the network. The integration of the current by the capacitor produces a voltage having the desired bathtub-waveform profile.
The invention has a number of advantages. The circuit is particularly simple since, for example in a preferred embodiment, it only requires two passive components in the form of a capacitor and a coil, and no active elements. As a result, the power loss is also minimized. The invention is particularly suitable for converting, in a simple manner, a known circuit for generating a parabolic correction voltage of the abovementioned first and third circuit types into a circuit for generating a bathtub-waveform correction voltage. Moreover, the circuit according to the invention does not require additional adjustment of components. Furthermore, in the case of the third circuit type described, the circuit makes it possible, in a simple manner, to effect blanking of the correction voltage during the vertical blanking interval.
The period duration of the resonant frequency of the network need not correspond exactly to the line trace period, but rather may, in particular, be somewhat longer than the line trace period. The respectively required form of the correction voltage can be achieved by corresponding dimensioning of the impedance of the network whilst adhering to the abovementioned tuning. In the case of a network in the form of an L/C series resonant circuit, smaller values for the capacitance and higher values for the conductance of the series circuit reduce the current in the series circuit and give the correction voltage generated a more parabolic profile. The maximum curve shaping in the direction of the bathtub shape is achieved if the peak value of the sinusoidal current in the series circuit reaches half the peak value of the current supplied by the secondary winding. Higher peak currents in the series circuit can lead to visibility of the oscillation frequency in the correction voltage on the screen. The respectively required amplitude of the correction voltage can be set by means of the turns ratio and, in the case of the third circuit type, by means of the air gap of the transformer.
In the dimensioning of the circuit according to the invention, under certain conditions the following difficulty may arise: in order to generate a dynamic correction voltage with an amplitude in the region of 1 kV, the capacitor connected in parallel with the secondary winding must have a value of the order of magnitude of a few hundred pF in the case of the abovementioned first and third circuit types, because only a small current having a peak value of about 10 mA flows in the secondary winding. Since the current in the LC series circuit must be less than or equal to half the sawtooth current, the result is a low capacitance of less than 500 pF for the capacitor and a relatively high value of more than 20 mH for the inductance. Such a high inductance is difficult to realize in practice. In order to eliminate this difficulty, in accordance with one development of the invention, the transformer is provided with a third winding, and the network, in particular the L/C series resonant circuit, is connected in parallel with this third winding. The third winding is preferably fixedly coupled to the secondary winding. Given a turns ratio of the secondary winding to the third winding of the order of magnitude of 6, in the case of a short circuit, the current in this third winding is six times higher than the current in the secondary winding W2, and the transformed capacitance of the capacitor across the third winding W3 is six times greater than the capacitance of the capacitor itself. With the low output impedance of the third winding, the embodiment of a corresponding L/C series resonant circuit is significantly simpler. For deflection at twice the line frequency, so-called 2H deflection, values of 6 nF for C and 2 mH for L can be used for the L/C series resonant circuit. This development of the invention can also advantageously be applied to the abovementioned third circuit type.
The function of the circuitxe2x80x94disclosed in WO 9306688xe2x80x94for blanking the correction voltage during the vertical flyback period is not adversely affected by the additional L/C resonant circuit according to the invention.